Telephone or like signaling system



Dec. 15,-1942.. ETALLv 2,305,137

' TELEPHONE OR LIKE SIGNALING SYSTEM Filed Oct. 7, 1940 2 Sheets-Sheet 1haZ FIGJ

- INVENTOR DAVlD LANGSFORD CLAY QgAYRLES J0 CUFF RD BARTLETT ATTORNEYEPHONE OR LIKE SIGNALING SYSTEM Dec. 15, 1942.

TEL

2 Sheets-Sheet 2 Filed Oct. 7. 1940 FIG. 2

DAVTD LANGSFORD CLAY CHARLES JOHN CLIFFORD BARTLETT ATTORNEY PatentedDec. 15, 1942 TELEPHONE R David Langsford LIKE SIGNALING SYSTEM Clay andCharles John Cliflord Bartlett, London, England, assignors to SiemensBrothers & 00. British company Application October Limited, London,England, a

7, 1940', Serial No. 360,038

In Great Britain October 10, 1939 11 Claims.

The present invention relates to telephone or like signaling systems andmore particularly to feeding-bridge circuits in or for such systems.

With the types of feeding-bridge circuit commonly employed in practice,trouble is often caused by undesired current surges produced bysignaling currents and especially by current surges produced during andas a result of impulse repetition. Such undesired current surges may beproduced at the bridge itself or may be generated at some other point.They are likely to be especially troublesome in systems where they mayreach a voice-frequency signal receiver of the kind liable to betemporarily paralysed by a strong input current.

The present invention has as one main object the provision of animproved feeding-bridge circuit wherein no surges of harmful magnitudecan be set up or transmitted as the result of impulse repetition. 7

According to one feature of the invention a feeding bridge circuit is ofthe kind in which the incoming and outgoing lines are coupled solely bythe transformer action of a repeating coil and an element or elementshaving non-linear resistance properties is or are connected to a windingor windings of the repeating coil in such a manner as to limit themagnitude of surges present at the bridge whilst having little eiiect oncommunication currents.

According to another feature of the invention a biassing voltage isapplied to the non-lineafrsistance element, the poling of the elementbeing such that in conjunction with the biassing voltage damping ofcommunication currents is negligible. This biassing voltage may beobtained by a connection to a separate source of current or it may beprovided by the drop in voltage occasioned by the flow of feed currentthrough the repeating coil winding to which the non-linear resistanceelement is connected. The term non-linear resistance element is hereinintended to include an element of which the resistance varies with thevoltage applied to it and also to an element which ofiers a differentresistance to the same voltage depending on the polarity of the voltage,i. e., it possesses unidirectional conductivity. An element of the firsttype is suitable for use in the case in which no biassing voltage isused and an element of the second type is employed in the case in whicha biassing voltage is applied to the element. A dry-plate rectifier oran element of a known compound of silicon carbide in which theresistance oiTered is high when the voltage due to communicationcurrents is applied but falls ing of the repeating coil to a lower valuedue to the application of a surge voltage higher than that produced bycommunication currents is suitable for use as a non-linear resistanceelement whether a biassing voltage is applied or not, it beingunderstood that the poling of an element possessing unidirectionalconductivity will be such that the element is suited to the connectionin which it is employed and that any biassing voltage applied to theelement will be such that the element does not become conductive to anygreat extent until the surge voltage exceeds that due to thecommunication currents.

In one Way of carrying out the invention the non-linear resistanceelements may be connected across the windings of the repeating coilconnected to the line and in the case of the windings on the incomingside of the bridge it is preferable to employ a balanced arrangement inwhich an element is connected across each of two windings of therepeatingcoil on the incoming side of the bridge. It will be realisedthat.

such a device is half-wave in action and will deal effectively withsurges arising at the bridge. It

' will also deal effectively with surges arising at a preceding pointprovided that the current at that point which gives rise to the surgeflows in such a direction that the initial polarity of the surge is thesame as that which would be produced by a surge at the bridge itself. Ina telephone system this can usually be arranged to be the case.

In another way of carrying out the invention the non-linear resistanceelement or elements may be connected across a winding of the repeatingcoil other than a line winding and hereinafter called a third windingirrespective of the number of windings connected to the lines. The thirdwinding may itself comprise two series connected windings. Twooppositely connected elements may be connected to the third windtoprovide an arrangement full-wave in its action and independent of thepolarity of the line wires.

In arranging for a bias on the elements when connected across the linewindings it may be found'desirable to include an external resistance,which should be non-inductive, in series with a repeating coil windingand to connect the element across this series connection. The resistanceshould be sufiiciently low in value to prevent its presence from undulyaffecting the efficiency of the bridge. The use of feed current toproduce the bias where the non-linear resistance elements are connectedto the line windings of the repeating coil has the advantage that it isthis current which, when interrupted, gives rise to the surge and by itsinterruption the bias on the nonlinear resistance element is removedrendering the arrangement more effective in dissipating the resultantsurge.

In the case in which non-linear resistance elements are connected to thethird winding of the repeating coil a suitable biassing voltage may beobtained either from a separate source of current or from suitableconnections made to the source of feed current. However by the choice ofa suitable element a biassing voltage may be dispensed with.

In an automatic telephone system the feeding bridge will include animpulse receiving relay on its incoming side. A relay having a lowinductance and operating at lay described in U.S. Patent No. ,992,6 g ad February 26, 1935, to E. J. Gachet, is suitable for use as an impulsereceiving relay and a contact of this relay may be arranged to repeatthe impulses in the outgoing side of the bridge or it may control animpulse repeating relay which may also be of the high speed type and acontact of this latter relay may be included in the outgoin line wires.

Specific embodiments of the invention will now be described by way ofexample with reference to the accompanyingdrawings. In the drawings Fig.1 shows diagrammatically an outgoing repeater with a feeding bridge inwhich incoming impulses are repeated as D. C. impulses over an outgoingcircuit, Figs. 2 and 3 show alternative forms of a similar repeater forrepeating D. C. impulses as impulses of voice frequency current in anoutgoing circuit, while Figs. 4, 5 and 6 show diagrammaticallymodifications of the arrangements at the feeding bridge that may beemployed.

Reference will first .be had toFig. 1. The relay set here shown is forconnection at the outgoing end of an inter-exchange junction whichserves as a link between automatic switching equipments at the twotelephone exchanges concerned, and as is usual in such relay sets it hasincoming positive, negative, and private wires for connection to themultiple of a group of selectors in the same exchange as itself andoutgoing positive and negative wires for connection to the two wires ofthe junction. In the drawings the incoming wires appear at the top onthe left, the private wires being designated P. The outgoing wiresappear at a corresponding position on the right, the private wireconnection shown in this case being intended simply for extension to adistributing frame for use for the purposes of local testing. Thecontacts designated T are contacts of a test jack. It will be noted thatunderneath the reference character for each relay there is a digitindicating the number of contacts on that relay. For example, relay G isdenoted by the letter G over the digit 3; this means that relay G hasthree contacts, numbered Ql, g2 and 93.

The principal functions of the relay set are to supply loop currentbackwards towards the originating end of the connection, to control theloop circuit which extends forwards to the succeeding exchange, and tocontrol the holding of the switches in the local exchange. In order thatit may properly perform these functions it includes a transmissionbridge .and must hence be adapted to repeat received impulses andsupervisory signals.

The relay set has a feeding bridge of the kind a high speed such astheree in which the incoming and outgoing circuits are coupled togetherby a so-called repeating coil RC. This repeating coil has two windings Iand III in the incoming circuit and two windings II and IV in theoutgoing circuit. Windings I and III are connected in series via a2-microfarad condenser QA and windings II and IV are connected in seriesvia a Z-microfarad condenser QB. Connected in series with windings I andIII respectively of the repeating coil RC and to the line areresistances RI and R2 and across the series connection of winding I andresistance Rl is connected a non-linear resistance element, suitably adry plate metal rectifier, SR1. A similar element SR2 is connectedacross the series connection of winding III and resistance R2. Theconnections on the incoming side of the feeding bridge are such that theincoming positive wire is normally connected to earth via winding I of ahigh-speed low-impedance impulsereceiving relay A and one element of atwo-element ballast resistance BRA, and the incoming negative wire isnormally connected to battery via winding II of relay A and the otherelement of BRA.

Condenser QB on the outgoing side of the bridge has two shunts connectedacross it. One of these shunts includes parallel connected rectifiersMRD polarised so as normally to allow current flow over the outgoingjunction during the initiation and setting up of a connection; the othershunt includes rectifier MR0 and relay D, rectifier MRC being sopolarised as to allow relay D to operate only when current in thejunction wires is reversed consequent on the reply of the calledsubscriber.

When the relay set is in circuit at an exchange and is free with itsapparatus at normal, it tests free to a searching selector by reason ofthe absence of earth on the incoming privatev Wire. When the relay setis seized for a call by such a selector, the high-speedimpulse-receiving relay A is operated in a circuit which includes theimpulsing loop which is controlling the setting up of the call. Theactual operating circuit is from earth over the lower element of theballast resistance BRA, winding I of relay A, winding III of therepeating coil RC resistance R2, back contact (M3, the incoming positivewire, the impulsing loop referred to the incoming negativewire, backcontact d012, resistance RI, winding I of the repeating coil, windingII- of relay A, the upper element of the ballast resistance BRA tobattery. With current flowing in this circuit the potential crosswinding I of the repeating coil and resistance R l in series is appliedto the nonlinear resistanceelement SRI biassing it so that currentthrough it at a voltage not higher than that due to speech in thecircuitis effectively blocked. A similar biassing potential is applied to theelement SR2 by the drop in winding III of the repeating coil'andresistance R2. Relay A is preferably a relay of the kind illustrated anddescribed in U. S. Patent No. 1,992,610, as before mentioned, and hasonly the one contact 12!, this being of the simple change-over type.Contact al on operation closes operating circuits for a high-speedimpulse-repeating relay AA and for the release relay B of the relay set,the circuit for relay AA comprising a rectifier MRB winding I of therelay, and resistance YB. The said impulse-repeating relay is a relay ofthe same kind as relay A and its sole contact aal on operation closes apoint in a calling loop across the outgoing positive and negative wires.A

elements SRI and SR2 providing a suitable path spark-quenchingcombination comprising a condenser QC in series with a resistance isconnected across the make portion of contact aal. It has been found thatthe presence of this combination exercises a beneficial effect onimpulse repetition. The release relay B is as usual a relay which isslow to release. On its operation as just described, its contact blconnects earth derived via back contact (M4 to the incoming privatewirefor engaging and holding purposes and contact b2 connects abattery-connected switching relay HA to this wire and thereby bringsabout the operation of this switching relay. The circuit of relay HAincludes a resistance YC, and the relay on operation locks up in acircuit over front contact hal and at contacts M12 and ha3 completes thecalling loop over the outgoing line wires. Contact b4 closes theoperating circuit of a slow releasing relay G. Relay G operates andperforms preparatory guarding operations. Consequent on the operation ofrelay HA, current flows over the outgoing junction and theparallel-connected rectifiers MRD but not through relay D owing to thepolarity of rectifier MRC. The rectifier MRB is to prevent the windingofrelay B from unduly increasing the release lag of 'relay AA by acting asa shunt. The connections. f the repeating coil RC are such that duringimpulse repetition the currents in all four windings assist each otherso far as the direct current magnetisation of the core is concerned.

Impulses are now received over the incoming line and are responded to byrelay A. At the first interruption in the line current, relay A releasesand at contact al opens the circuit for relay AA. Relay AA releases andat contact aal opens the outgoing loop to repeat the impulse break andcloses a short-circuit across the outgoing side of the feeding bridge.Consequent on the closure of back contact al a circuit is closed overcontacts (1125 and b3 and the two windings of relay C in series and thatrelay operates. Contact cl short circuits winding I of the .relay torender it slow in releasing and contact 02 connects a low resistanceshunt across the outgoing side of the bridge. Subsequent receivedimpulses are repeated by relay AA over the junction.

In consequence of the opening of the incomingline wires the energy inthe windings of the repeating coil tends to dissipate itself in the formof a surge. As regards the energy produced in the windings I and III ofthe repeating coil, a path for the surge current is provided over theelements SR! and SR2 the bias on which was removed consequent on thecessation of current round the loop. The potential drop across winding Iand resistance RI and winding III and resistance R2 respectively is suchas to cause a current to flow through the elements SR! and SR2 and thevoltage across the elements is reversed in polarity and the surgedissipates itself so that surge current of any magnitude is pre- Ventedfrom passing through the repeating coil to the outgoing line. Bycontrolling the bias on the non-linear resistance elements by the feedcurrent which on interruption gives rise to the surge, the bias isremoved on interruption of the feed current and a rapid dissipation ofthe surge energy results. It will be noted also that the production of asurge of the same polarity at a preceding point will also be preventedfrom passing through the bridge without attenuation, the

for the dissipation of the surge energy.

At the end of each train of impulses relay C releases removing the shuntfrom the outgoing side of the bridge. After the connection has beencompletely set up and the called party replies, relay D is operated bycurrent reversal over the junction. The closing of contact dl initiatesthe slow operation of an answering supervisory relay DD having a longoperation lag, and contact d2 completes a circuit for a holding windingII of the impulse-repeating relay AA. Relay DD on operation istemporarily locked up in a circuit over contacts old! and M and owing toits slow operating property serves to ensure that metering is noteffected unless the reversed current condition in the outgoing lineWires persists for a reasonable time. The changing over of contact dollinitiates the slow release of the metering relay G, and contacts (1012and 11013 effect a reversal of current in the incoming line wires torepeat the answering supervisory signal in case this should benecessary. Contact ddd connects booster battery potential to theincoming private wire in place of the usual holding earth, so that ifthe calling party is a subscriber belonging to the exchange in which therelay set is situated, the meter of this party is operated in serieswith re-- sistanc YD. When the slow release of relay G is completed.contact gl opens the holding circuit of relay AA and contact 92 closesan operating circuit for a control relay H which circuit includescontacts 134, dl and hl. On the operation of relay H, make-before-breakcontact hl renders the circuit of the relay independent of the conditionof relays D and G, contacts I22 and M restore the usual holdingcondition on the incoming private wire, and contact M opens a holdingcircuit for relay DD. Release of the switching relays connected to theprivate Wire is prevented during the changeover period by the presenceof holding earth applied over rectifier MRA which opposes flow ofbooster battery current. The condition of th circuit immediatelysubsequent to metering is that relays A, AA, B, HA, D, DD, and H areoperated.

If at the end of the call the calling party clears first, then relay Areleases, this release causing the immediate release of relay AA andinitiating the slow release of the release relay B. The falling back ofcontact aal opens the loop across the outgoing junction wires, releasingrelay D and initiating the release of the apparatus at the distant endof the junction. The opening of contact all releases relay DD. Thefalling back of contact ddl operates the metering relay G, and thefalling back of contact dd5 operates the dialling relay C. Contact g3closes an alternative circuit for relay C. Contact ci short-circuits thelow-resistance winding I of relay C to render the relay slow to release.On the subsequent completion of the slow release of the release relay B,the opening of contact bl temporarily removes the holding earth'from theincoming private wire to release the switching relay HA and the selectorswitching relays held locked to this earth (thus causing the immediatecommencement of the release of the apparatus preceding the relay set),and the opening of contact 194 releases relay H andinitiates theslowrelease of relay G. The switching relay HA is arranged to release rathermore slowly than the said selector switching relays, and when it doesrelease, at contact hal for again it completes a circuit over contact 03earthing the incoming privat Wire to guard the relay setfrom seizure byasearching selector. On the completion ofthe slowrelease of relay G, theopening of contact 913 initiates the slow release of relay C. On thecompletion in turn of the slow release of relay C, the guarding earth isdisconnected from the incoming private wire and the relay set is readyfor use on another call. Owing to the sequence of operations justdescribed, the apparatus at the distant end of the junction is givenample time to release before the relay set tests free, while thepreceding apparatus is not held for the whole of the guarding period.

If the called party clears before the calling party, the polarisedback-bridge relay D is in accordance with the hitherto usual practice released by the usual reversal of current in the line wires of thejunction, and at its contact all releases the answering supervisoryrelay DD.

The falling back of contact (1015 operates the metering relay G, and thefalling back of contacts dd? and ddt eiiects a reversal of current inthe incoming line wires to repeat'the clearing signal in case thisshould be necessary. When the calling party clears, relays A and AArelease in turn, relay C is operated over back contact ai, and the slowrelease of relay B is initiated. The falling back of contact cal opensthe loop across the outgoing junction wires, initiating the release ofthe apparatus at the distant end of the junction. On the subsequentcompletion of the slow release of the release relay B, release of theconnection proceeds in the same manner as in the case considered in thepreceding paragraph.

If the called party is not free when called, then busy tone current isreceived over the junction and passes via the repeating coil bridge tothe calling party. When the calling party clears, relays A and AArelease in turn, and release of the connection proceeds in a mannersimilar to that just described.

The feeding bridge shown. in Fig 2- is suitable for use in a system inwhich incoming D. C. impulses are to be repeated as impulses of voicefrequency current. bridge is arranged in a manner similar to that shownin and described for Fig. 1. The repeating coil windings on theoutgoings'ide of the bridge are connected to the outgoing junction overthe back contact of a change-over spring set of a dialing relay and thefront contact of a change over spring set of an impulse repeating relayAA. The front contact of the change-over spring set of the dialing relayconnects a 690 ohm. termination across the line and the back contact ofthe changeover spring set of the impulse repeating relay connects thesecondary winding of a tone ransformer TTto the line. The primarywinding of the tone transformer is connected to a source of voicefrequency current. The impulse repeating relay corresponds to the relaydesignated AA in Fig. 1 and the dialing recorresponds to the relaydesignated C in Fig. 1 and may be operated and controlled in well knownmanner. When the relay set is taken into use relay AA operates and atcontact aal disconnects the tone transformer from the line wires, itunderstood that normally the line is held open by contacts of aswitching relay which is operated to close the line circuit when therelay set is taken into use. On receipt of the first impulse break.relays A and AA release and by means of a previously prepared circuitthe dialing relay operated. Relay AA releases and connects the tonetransformer to the line so that The incoming side of the for theiduration of the break period a pulseof voice :irequency current istransmitted. When the impulse repeating relay again operates, the GOOchmtermination is connected to the line over contact ci. The dialling relayremains operated throughout the duration of animpulse train and releasesat the end of the trainin knowirrnanner.

The action of the non-linear resistance elements'SR! and SR2 is the sameas that described in connection with Fig. 1. With the arrangementsdescribedparalysation of the voice frequencycurrent receiver due to thetransmission of surges can be reducedto a n'egligibleamount. Like thecasev of Fig. 1 the surge limiting action is half-wave inits action andcan deal'with surges'arising at the bridge and with surges arising at apreceding point if the polarity is'the same. A surge limiting resultmay'also be obtained by connecting a non-linear resistance elementacross the repeating coil windings on the outgoing side of the bridge.The element may be a rectifier and have a suitable biassing voltageapplied to it. The magnitude of the biassing voltage and'the poling ofthe element would be so arranged that a surge arising at the bridgewould be shunted away from the line Whilst communication currents wouldsuffer little attenuation. Such an arrangement is illustrateddiagrammatically in Fig. 3 in which the element is designated SR and thesource of biassing voltage V. The remainder of the apparatus illustratedin this figure is similar to the-correspondingly designated apparatus inFig. 2. An advantage of this arrangement lies in the fact that itbecomes effective before the dialling relay possessing contact cloperates and therefore is effective to deal with a surge arising fromthe transmission of the first impulse of a train.

As an alternativ to the arrangement illustratedin Fig. 3 the element SRmay be of a type which does not necessitate a biassing voltage, theresistance of the element under the communication current conditionbeing sufiicientlyhigh to prevent appreciable damping of speech currentsand under surge conditions sufficiently low for the rapid dissipation ofthe surge energy.

It has been found that with repeating'coil windings of 10 ohms each andnon-inductive resistances RI and R2 also of 10 ohms each a suitable biasfor a dry plate rectifier for use as SR! and SR2 is obtained on a lineup to 800 ohms loop resistance, the value of bias to beprovided-dependin on thecharacteristics of the devices SR! and SR2.

Other arrangements for providing the surge limitation coming within theinvention are illustrated in Figs. 4, 5 and 6. In these figures, whichare only diagrammatic, the repeating coil is shown with two linewindings I and II and a third winding III to which'non-linear resistanceelements are connected. The arrangements shown in these figuresarefull-wave in action. In Fig. 4 two rectifiers SR! and SR2 withlike-faces connected to the ends of the third winding-of the repeatingcoil have their other faces connected together and to one end of asource of biassin voltage V the other end of the source being connectedto the mid point of the third winding III. It will be seen that when thevoltage induced inwinding III exceeds the voltage of source V andopposes it the'bias on the rectifiers is overcom and they becomeconductive and provide a low resistance shunt in which the surge may,dissipate itself.

The bias provided by the source V is such that no appreciable shuntingeffect is present when communication currents are flowing in therepeating coil. It will be understood by those skilled in the art thateach non-linear resistance element may be associated in oposite senseswith a separate winding, the result being equivalent to the singlewinding with mid point connection shown in the figure. Furthermore, bythe choice of a suitable element the element may be connected across thethird winding of the repeating coil without the application of a,biassing voltage.

The arrangements illustrated in Figs. and 6 are similar in their actionto those illustrated in Fig. 4 the source of biassing voltage being apotentiometer P'connected across a battery V. These arrangements obviatethe necessity for the use of a separate biassing battery and in atelephone system V may be the exchange battery.

It will be understood that a half-wave action may be had by theconnection of only one rectifier to the winding 'III and the applicationof a suitable biassing voltage but the full-Wave action deals not onlywith surges arising at the bridge but also with surges arising at apreceding point whether the polarity is the same or not.

We claim:

1. In combination, a circuit including resistance, means for impressingaudio frequency upon said circuit, a source of direct current in saidcircuit biassing same to cause the fiow of current over said resistancealways to be in a particular direction even when said resistanceistraversed by said audio frequency, a rectifier connected in shunt tosaid resistance, whereby the voltage drop in said resistance due tocurrent flowing therethrough produces a difference of potential acrosssaid rectifier, said rectifier so poled with respect to said voltagedrop that it will resist current flow through itself whenever thecurrent flow through said resistance is in said particular direction,said rectifier becoming conductive whenever an abnormal potential insaid circuit tends to reverse the current flow through said resistance,thereby to limit the magnitude of surges through said resistance.

2. In combination, a circuit including an inductance, means forimpressing audio frequency upon said circuit, a source of direct currentin said circuit biassing same to cause the now of current through saidinductance always to be in a particular direction even when saidinductance is traversed by audio frequency, a rectifier connected acrosssaid inductance, whereby the voltage drop in said inductance due tocurrent flowing therethrough produces a difference of potential acrosssaid rectifier, said rectifier so poled with respect to said voltagedrop that it will resist current flow through itself whenever thecurrent flow through said inductance is in said particular direction,said rectifier becoming conductive due to the surge from said inductancein the event the current flow from said source is interrupted.

3. In a telephone or like system, a communication line divided into twosections, a repeating coil inductively linking said two line sections, asource of direct current connected to one of said line sections over acircuit including a winding of said coil, said source efiective tomaintain current flowing through said circuit in a particular directioneven when said winding of the repeating coil is traversed by speechcurrents, a rectifier connected in shunt to said circuit, whereby thevoltage drop in said circuit due to said current flowing therethroughproduces a difference of potential across said rectifier, said rectifierso poled with respect to said voltage drop that it is substantiallynon-conductive so long as said current continues to flow in saidparticular direction through said circuit, said rectifier becomingconductive whenever an abnormal potential on said line tends/to reversethe direction of current flow through said circuit, thereby to limit themagnitude of surges through said circuit.

4. In a telephone or like system, a battery feed bridge comprising asource of direct current, a relay having two windings, a repeating coilhaving a plurality of windings, two non-inductive resistors,-a linehaving two conductors, means connecting one terminal of said source toone of said conductors over a circuit including one winding of saidrelay, one winding of said coil and one of said resistors in series,means connecting the other terminal of said source to the other of saidconductors over a circuit including the other winding of said relay,another Winding of said coil and the other of said resistors in series,a rectifier shunting that portion of said first circuit including saidone winding of said coil and. said one resistor, another rectifiershunting that portion of said second circuit including said otherwinding of said coil and said other resistor, each of said rectifiersbeing so poled that they offer high resistance to the flow of currentfrom said source when said two conductors are connected together.

5. Ina telephone or like system, a line divided into two sections, arepeating coil inductively linking said two line sections, a relay, asource of direct current, means connecting said source to one of saidline sections over a circuit including said relay and a winding of saidrepeating coil in series, whereby a normal current flow is establishedover said relay, said winding and said one line section, said relayresponsive to interruptions of said normal current flow, the inductanceof said one line section and of said relay tending to cause current tosurge through said winding in a direction opposite to said normalcurrent flow upon each said interruption of said normal current flow,and a non-linear resistance connected in bridge to said winding andeffective to limit the magnitude of said current surges through saidwinding.

6. In a system as claimed in claim 5, means controlled by said relay forrepeating said interruptions as impulses over the other section of saidline, and other means controlled by said relay upon commencement of saidrepetition for short-circuiting the winding of said repeating coilconnected to said other line section.

7. In a telephone or like system wherein a com nection may be extendedfrom certain equipment to other equipment at will, wherein said otherequipment thereupon is effective to impress a direct current potentialupon two conductors of said extended connection, and wherein said otherequipment thereafter is effective to reverse the polarity of thepotential connected to said conductors for supervisory purposes, a firstcircuit bridging said two conductors in said certain equipment, saidcircuit containing a rectifier poled to permit current to flow over saidcircuit only when the potential impressed upon the conductors at saidother equipment is of a certain polarity, a second circuit bridging saidtwo conductors in said certain equipment, said second circuit containinga rectifier poled to permit currentrto flow over saidisecondcircuitonly. when the-potential impressed upon the conductors at said otherequipment is of a difierent-polarity, and a relay included in one ofsaid circuits and operated whenever there is a current flow over thatcircuit due to the potential-impressedupen the conductors atsaidotherequipment.

8. In a telephone or like system, a communicationline divided into twosections, a repeating coil having a different winding connected to eachof said line sections thereby to inductively coupie said two sections,said repeating coil also'having a winding other than those connected tosaid two line sections, a rectifier, a source of direct current,andmeans connecting said source, said rectifier and at least a portionof said other winding in a series circuit, said rectifier being so poledin said circuit that it presentsahigh resistance to the flow of currentover said circuit from said source.

9. A system such as claimed in claim 8, wherein the potential impressedupon'said rectifier by said source is greater than that impressedthereupon due to the currents induced'in said'other winding when speechcurrents are present on said communication line.

10. In 'a telephone or like system, a communication line divided intotwo sections, a repeating coi1 having a diilerent winding connected toeach of said line sections thereby to inductively couple said twosections, said repeating coil also having a winding other than thoseconnected to said two line sections, a source of direct current havingone terminal connected'to the center point of said other winding, arectifier connected between the other terminal of said source and oneend of said other winding, and a second sectifier connected betwen saidother terminal and the other end of said other winding, each of saidrectifiers being so poled that it presents high resistance to the fiowof current over half of said other winding from said source.

11. In a telephone or like system, a communication line divided into twosections, a repeating coil having a different winding connected to eachof said line sections thereby to inductively couple said two sections,said repeating coil also having a winding other than those connected tosaid two line sections, a source of direct current, a voltage dividerconnected across said source, means connecting the mid-point ofsaid'divider'to one end of said other winding, a rectifier connectedbetween the other end of said other winding and one terminal ofsaid-source, a second rectifier connected between said other end ofother winding and the other terminal of said source, each of saidrectifiers so poled that it presents high resistance to the flow ofcurrent from said source over said other winding and one half of saiddivider.

DAVID LANGSFORD CLAY.v CHARLES JOHN CLIFFORD BARTLETT.

